1. Field of the Invention
This invention relates to wireless communication systems, and more particularly to a method and apparatus for efficiently synchronizing MAC and physical communication protocol layers of a wireless communication system.
2. Description of related Art
As described in the commonly assigned related co-pending application No. Ser. 08/974,376, a wireless communication system facilitates two-way communication between a plurality of subscriber radio stations or subscriber units (fixed and portable) and a fixed network infrastructure. Exemplary communication systems include mobile cellular telephone systems, personal communication systems (PCS), and cordless telephones. The key objective of these wireless communication systems is to provide communication channels on demand between the plurality of subscriber units and their respective base stations in order to connect a subscriber unit user with the fixed network infrastructure (usually a wire-line system). In the wireless systems having multiple access schemes a time xe2x80x9cframexe2x80x9d is used as the basic information transmission unit. Each frame is sub-divided into a plurality of time slots. Some time slots are used for control purposes and some for information transfer. Subscriber units typically communicate with a selected base station using a xe2x80x9cduplexingxe2x80x9d scheme thus allowing for the exchange of information in both directions of connection.
Transmissions from the base station to the subscriber unit are commonly referred to as xe2x80x9cdownlinkxe2x80x9d transmissions. Transmissions from the subscriber unit to the base station are commonly referred to as xe2x80x9cuplinkxe2x80x9d transmissions. Depending upon the design criteria of a given system, the prior art wireless communication systems have typically used either time division duplexing (TDD) or frequency division duplexing (FDD) methods to facilitate the exchange of information between the base station and the subscriber units. Both the TDD and FDD duplexing schemes are well known in the art.
Recently, wideband or xe2x80x9cbroadbandxe2x80x9d wireless communications networks have been proposed for delivery of enhanced broadband services such as voice, data and video. The broadband wireless communication system facilitates two-way communication between a plurality of base stations and a plurality of fixed subscriber stations or Customer Premises Equipment (CPE). One exemplary broadband wireless communication system is described in the co-pending application Ser. No. 08/974,376 and is shown in the block diagram of FIG. 1. As shown in FIG. 1, the exemplary broadband wireless communication system 100 includes a plurality of cells 102. Each cell 102 contains an associated cell site 104 that primarily includes a base station 106 and an active antenna array 108. Each cell 102 provides wireless connectivity between the cell""s base station 106 and a plurality of customer premises equipment (CPE) 110 positioned at fixed customer sites 112 throughout the coverage area of the cell 102. The users of the system 100 may include both residential and business customers. Consequently, the users of the system have different and varying usage and bandwidth requirement needs. Each cell may service several hundred or more residential and business CPEs.
The broadband wireless communication system 100 of FIG. 1 provides true xe2x80x9cbandwidth-on-demandxe2x80x9d to the plurality of CPEs 110. CPEs 110 request bandwidth allocations from their respective base stations 106 based upon the type and quality of services requested by the customers served by the CPEs. Different broadband services have different bandwidth and latency requirements. The type and quality of services available to the customers are variable and selectable. The amount of bandwidth dedicated to a given service is determined by the information rate and the quality of service required by that service (and also taking into account bandwidth availability and other system parameters). For example, T1-type continuous data services typically require a great deal of bandwidth having well-controlled delivery latency. Until terminated, these services require constant bandwidth allocation for each frame. In contrast, certain types of data services such as Internet protocol data services (TCP/IP) are bursty, often idle (which at any one instant may require zero bandwidth), and are relatively insensitive to delay variations when active. The base station media access control (xe2x80x9cMACxe2x80x9d) allocates available bandwidth on a physical channel on the uplink and the downlink. Within the uplink and downlink sub-frames, the base station MAC allocates the available bandwidth between the various services depending upon the priorities and rules imposed by their quality of service (xe2x80x9cQoSxe2x80x9d). The MAC transports data between a MAC xe2x80x9clayerxe2x80x9d (information higher layers such as TCP/IP) and a xe2x80x9cphysical layerxe2x80x9d (information on the physical channel).
Due to the wide variety of CPE service requirements, and due to the large number of CPEs serviced by any one base station, the bandwidth allocation process in a broadband wireless communication system such as that shown in FIG. 1 can become burdensome and complex. This is especially true with regard to rapidly transporting data while maintaining synchronization between the MAC and physical communication protocol layers. Base stations transport many different data types (e.g. T1 and TCP/IP) between the MAC and physical layers through the use of data protocols. One objective of a communication protocol is to efficiently transport data between the MAC and physical layers. A communication protocol must balance the need for transmitting data at maximum bandwidth at any given time against the need for maintaining synchronization between the MAC and physical layers when the data is lost during transportation.
Prior art communication protocols have been proposed for transporting data in a wireless communication system. One prior art communication protocol teaches a system for transporting MAC messages to the physical layer using variable length data packets comprising headers and payloads. A payload contains data for a MAC message data type (e.g., T1 and TCP/IP). In the prior art, a header starts at a physical layer boundary and provides the wireless communication system with information such as the length of the payload and the location of the next data packet. Typically, the communication protocol provides adequate bandwidth usage via the variable length data packets. However, this type of protocol provides poor synchronization between the MAC and physical layers because when the system loses a header the protocol overlooks all of the subsequent data until it finds the next header at the beginning of the physical layer boundary. The system then begins using data from that physical layer boundary. Thus, the variable length data packet protocol loses a relatively large amount of received data (i.e., the data received between the lost header and the next physical boundary). It is therefore an inefficient communication protocol for use in a wireless communication system.
Another prior art protocol teaches a system for transporting MAC messages using fixed length data packets. In accordance with these systems, a message always begins at a fixed position relative to the other messages. When the system loses a part of a message, the protocol only loses that one message because it can find the next message at the next fixed position. Thus, the fixed length data packet protocol provides adequate MAC to physical layer synchronization. However, the fixed length data packet protocol provides poor bandwidth usage because a fixed length data packet must be sufficiently large to accommodate the largest message from any given data type. As most messages are much smaller than the largest message, the fixed length packet protocol typically wastes a large amount of bandwidth on a regular basis.
Therefore, a need exists for a data transportation and synchronization method and apparatus for efficiently transporting data between the MAC and physical layers in a wireless communication system. The data transportation and synchronization method and apparatus should accommodate an arbitrarily large number of CPEs generating frequent and varying bandwidth allocation requests on the uplink of a wireless communication system. Such a data transportation and synchronization method and apparatus should be efficient in terms of the amount of bandwidth consumed by the messages exchanged between the plurality of base stations and the plurality of CPEs in both the uplink and downlink. In addition, the data transportation and synchronization method and apparatus should rapidly synchronize to the next data message when a part of a message is lost as to prevent a large loss in data. The present invention provides such a data transportation and synchronization method and apparatus.
The present invention is a novel method and apparatus for efficiently transporting and synchronizing data between the MAC and physical layers in a wireless communication system. The method and apparatus reduces the amount of unused bandwidth in a wireless communication system. The present invention advantageously synchronizes rapidly to the next data message when a data message header is lost across the data or the air link. The present invention utilizes a combination of data formats and a data transportation technique to efficiently transport data in a communication system.
In the preferred embodiment of the present invention, the data format for a MAC packet is preferably variable in length. Depending on the length of the MAC packet to be transported, the present invention either fragments or concatenates the MAC packet during mapping to the physical layer. The physical layer contains Transmission Convergence/Physical (xe2x80x9cTC/PHYxe2x80x9d) packets having fixed length payloads. The present invention includes a novel technique for transporting and mapping variable length MAC packets into TC/PHY packets.
In accordance with the present invention, the present inventive method initiates the data transportation and synchronization technique by obtaining a MAC packet. The method determines whether the MAC packet is longer than the available bits in the payload of the present TC/PHY packet. If so, the method proceeds to fragment the MAC packet and map the fragments into successive TC/PHY packets. The present inventive method and apparatus may be adapted for use in either an FDD or TDD communication system. When used in a TDD system, the successive TC/PHY packets are preferably transmitted back-to-back within the same TDD frame.
If the method determines that the MAC packet is shorter than the available bits in the payload of the present TC/PHY packet, the method proceeds to map the MAC packet. After mapping the MAC packet to the TC/PHY packet the method determines whether the next MAC packet should be mapped with the previous MAC packet in the TC/PHY packet. The method will concatenate the next and previous MAC packets unless either of the following two conditions apply. The first condition is a change in modulation on the downlink. Upon such a change, the first packet at the new modulation starts in a new TC/PHY packet following a modulation transition gap (MTG). The second condition is a change in CPE on the uplink. Upon such a change, the first packet from the next CPE starts in a new TC/PHY packet following a CPE transition gap (CTG). If neither condition applies, the method maps the next and previous MAC packet in the same TC/PHY packet in the manner described above.